An IGBT (insulated gate bipolar transistor) can be integrated with a diode to form a semiconductor device such as a reverse conducting IGBT. In such a semiconductor device, on the front surface side of a semiconductor layer, a MOS structure (e.g., a structure including an emitter region, a collector region, and a gate electrode provided on the emitter region via an insulating film) is provided. On the back surface side of the semiconductor layer, a p-type contact region as a collector region of the IGBT and an n-type contact region as a cathode region of the diode are provided. A back surface electrode is connected to the p-type contact region and the n-type contact region.
In forming the p-type contact region and the n-type contact region, typically, p-type impurity and n-type impurity, respectively, are selectively implanted into the back surface of the semiconductor layer and activated by heating. The back surface of the semiconductor layer may be polished to thin the semiconductor device. Activation of the p-type contact region and the n-type contact region is often performed by interrupting the MOS manufacturing process to avoid thermal damage to the MOS structure formed on the front surface side of the semiconductor layer.
However, in the MOS manufacturing process, if the thickness of the semiconductor layer is thinned, cracking and chipping may occur in the semiconductor layer during the process. Furthermore, if the thickness of the semiconductor layer is varied by polishing during the MOS manufacturing process, dedicated systems are required for different thicknesses before and after the polishing. This incurs the increase of manufacturing cost.
Furthermore, for instance, if the projection range of impurity of the n-type contact region is too shallow, n-type impurity may scatter to the p-type contact region during activation. This may increase the contact resistance between each contact region and the back surface electrode and impair the device characteristics. Conversely, if the projection range of impurity is too deep, activation of impurity in each contact region may end up in failure and again impair the device characteristics.